FCGetMetricSignature[] returns the signature of the
Minkowski metric used when working with Cartesian objects, such as
CartesianPair, CartesianIndex,
CartesianMomentum etc.
{1,-1} corresponds to (1,-1,-1,-1) and {-1,1} means
(-1, 1, 1, 1).
Overview, FCSetMetricSignature.
FCGetMetricSignature[]\{1,-1\}